San Francisco voters in 2003 made it illegal for city officials to get paid to try to influence the decisions of their fellow city officers.

The idea was to guard against conflicts of interest and root out favoritism or undue influence.

It looks like not everyone got the memo.

Claudine Cheng, a long-serving commissioner on the city's oversight board for Treasure Island development, has quietly resigned and is now the subject of a complaint for violating just that city rule.

Cheng, a member of the Treasure Island Development Authority board of directors almost continuously since then-Mayor Willie Brownappointed her in 2001, appears to have tried to influence city decisions in at least two areas while serving on that board, city records show.

One was approval for a bitterly contested Pet Food Express store on Lombard Street, the other a zoning change to allow a popular North Beach breakfast and lunch spot to expand into a building zoned mainly for a neighborhood grocery store.

Cheng has a lengthy record of volunteering for community programs helping children, the homeless and others. She sent an e-mail saying she is out of town taking care of her ailing mother. She did not address the allegations and said instead her "sole focus" is on her mom. In her resignation letter to Mayor Ed Lee, Cheng wrote that "new roles" in her consulting business legally prevented her from serving as a city official.

"Unfortunately, as I have taken on new roles in my law practice and professional responsibilities, I am precluded by ethics rules from serving in the capacity of an appointed officer in the city," Cheng wrote in the Aug. 1 letter.

Records, however, appear to indicate the questionable activity predated her resignation.

Cheng, an attorney who runs her own consulting business and started her legal career at Brown's law firm when the now-Chronicle columnist was Assembly speaker, has been e-mailing Board of Supervisors PresidentDavid Chiufor months seeking a legislative amendment to make a zoning change to allow for North Beach favorite Mama's to expand into a second location, e-mails obtained through a public records request show.

The site on the corner of Vallejo Street and Columbus Avenue used to house the Piazza Market.

Cheng wrote to Chiu in a March 22 e-mail: "We look forward to moving onto the next steps, which based upon my understanding would involve your discussion with the city attorney's and planning staff for purpose of drafting the appropriate language for the legislative amendment."

She e-mailed him as recently as July 9, writing, "The period between now and September is the perfect window of time to iron out the language of the amendment," records show.

Cheng at the time was not a registered lobbyist, city records show, even though the city defines a lobbyist as someone who receives or is promised at least $3,000 within three consecutive months "for the purpose of influencing local legislative or administrative action."

There is an exception for someone "performing a duty or service that can be performed only by an attorney, an architect, or a professional engineer," but the correspondence did not involve litigation or similar legal matters, the records provided show. The day Chiu's office turned over those public records about Cheng's efforts - July 24 - she registered as a lobbyist, city Ethics Commission records show. About a week later, she resigned.

Chiu said he didn't know if Cheng was being paid to push for the legislation.

"Claudine has worked with my office on many community causes as a volunteer," Chiu said. "In this instance, I have no idea if she was being paid."

Cheng's financial disclosure forms filed in May, though, indicate she earned between $10,000 and $100,000 for her consulting business in 2012, with the only client providing more than $10,000 listed as 627 Vallejo LLC. Property records show that's the owner of the former Piazza Market site that Mama's would like to move into. Mike Sanchez, one of the owners of Mama's, also said Cheng was working for the property owner.

In addition, Cheng is currently a paid consultant for Pet Food Express, and was on June 10, said Michael Levy, the company's founder and co-owner.

Katz testified in favor of Pet Food Express at the meeting, as did Cheng.

"There's a reason this is illegal. It's basically insider trading," said former Board of Supervisors President Aaron Peskin, who opposes the expansion of Pet Food Express there and filed complaints against Cheng with the Ethics Commission and the Fair Political Practices Commission. "She is resigning under a cloud because she has lobbied without a license and has violated San Francisco's ethics law repeatedly. A lawyer should know better."

Ethics Commission head John St. Croixsaid he was prohibited by law from commenting on any active investigation.

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